Cathode ray tube and target



Nov. 13, 1956 s. GRAY CATHODE RAY TUBE AND TARGET Filed March 25, 1953INVENTOR.

Jjdlzey Gray A TTOR NE 1 CATHODE RAY TUBE AND TARGET Sidney Gray, NewBrunswick, N. J., assignor to Radio Corporation of America, acorporation of Delaware Application March 25, 1953, Serial No. 344,616

17 Claims. (Cl. 313-65) My present invention relates to cathode rayelectron tubes and targets therefor. More particularly my inventionrelates to tubes in which a varying electron charge image is formed in asingle plane and which are suitable for use as color television pickuptubes.

Such tubes have been made in which an electron beam scans a target withall of the scanning occurring in a single plane. The scanned surface ofray sensitive material, in the form of spaced elements or strips or inthe form of a continuous coating, is usually capacitively coupled to asignal plate. The signal plate is segmented as for example in the formof strips which are of conductive material and extremely thin. Often thesignal plate strips are so thin as to be transparent. Furthermore, thestrips are very narrow and arranged in close spaced relation. Thetransparent conductive strips are formed on color filters which extendacross the target while the ray sensitive material, which may bephoto-conductive, is laid down on the transparent conductive strips. Thecolor filters are interference filters such as may be formed fromsuccessive layers of suitable material; one having a high and the othera low index of refraction. Conveniently, the color filters andtransparent conductive strips forming the segmented signal plate areformed by evaporating the materials through a wire masking grill asdescribed in the U. S. Patent 2,745,773 of P. K. Weimer, Serial No.344,498, filed simultaneously herewith and assigned to the assignee ofthis application. The color filters are of insulating material andarranged in a repetitive pattern of groups of colors. The transparentconductive strips on the filters of the same color are connected to busbars, usually at both ends thereof, so as to form interlaced sets with aset for each color.

When object light falls on such a target, the light passes through thefilters and transparent conductive strips into the photo-conductivematerial. Assuming red, green and blue filters, the scanned surface ofthe ray sensitive material assumes a charge pattern in the absence ofthe electron beam in accordance with the color components and intensityof the object light. Areas receiving light through the red filtersrespond only when the object light includes red and is substantiallyinsensitive to green or blue. The electron beam acts to return theelemental area it impinges upon to what may be termed its dischargedcondition and simultaneously a voltage pulse appears on the transparentconductive strip. Ideally, the voltage pulse sees an infinite impedanceexcept along the conductive strip to the bus bars connecting that stripto the output circuit.

As pointed out above the transparent conductive strips are closelyspaced and in view of the interlaced arrangement described a conductivestrip for one color has closely adjacent thereto a strip of each of theother colors. Because of this close spacing, as well as for otherreasons including the scattering of some conductive material into thespaces, the impedance between adjacent transparent conductive strips isobjectionably low. In fact, the re- States Patent active impedancebetween the sets of transparent conductive strips is fairly low.

In practice it is proven difiicult to make such tubes which produce highfidelity color video signals. Furthermore, the pictures obtained at thecolor television receiver are often streaky, while in portions of thepicture colors which should be uniform are non-uniform.

I have found that color fidelity may be improved, cross talk between thedifferent colors as well as streaks in the picture obtained may besubstantially eliminated, by improving the connection between each ofthe transparent conductive strips and the bus bars connected thereto. 1have found that pickup tubes while seemingly operating satisfactorilycause the defects appearing at the receiver because of high impedanceconnections between the conductive strips and the bus bars. I attributethe cause of the foregoing ditficulties to the fact that at least someof the transparent conductive strips are connected to their bus barsthrough a relatively high impedance. The impedance is sufiiciently highthat the voltage pulses appearing on the strips travel to adjacentstrips almost as well as they travel along 'a strip.

It is therefore a principal object of my invention to provide animproved cathode ray tube, particularly suitable as a pickup tube forcolor television, capable of pro ducing color video signals of highfidelity and uniformity.

Another object of my invention is to provide an improved colortelevision pickup tube having sets of interlaced transparent conductivestrips having uniformly low impedance connections to their respectiveleads or bus bars.

A more specific object is the provision of a target for cathode raytubes particulary suitable for a color television pickup tube in whichthe conductive signal strips are connected to their respective bus barsby uniformly low impedance connections.

In accordance with my invention it provide a target particularlysuitable for color television pickup tubes in which each transparentconductive signal strip in a set of signal strips is connected to itsrespective bus bar by means of a conductive material which reduces theimpedance between the strip and its bus: bar. Preferably, I utilize amaterial which not only reduces the reactive impedance but which rendersintervening nonconductive films conductive.

Other objects and features of my invention will be apparent from theaccompanying detailed description thereof and the drawing in which:

Figure 1 is a sectional view of a pickup tube, being illustrative of oneparticular type of cathode ray tube, suitable for color television;

Figure 2 is a plan view of the target of the tube shown in Figure I,greatly enlarged and not drawn to scale for convenience;

Figure 3 is a fragmentary sectional view of the target shown in Figure2.

For the purpose of illustrating my invention I will now describe thesame in connection with photo-conductive pickup tube 10 shown inFigure 1. However, it should be clearly understood that my invention isequally ap plicable to other types of tubes and in particular to othertypes of pickup tubes suitable for color television. Pickup tube 10comprises an envelope 11 having the usual electron gun 12 mountedtherein. As is well known, the electron gun 12 contains a cathode andvarious accelerating and focusing electrodes. A final accelerating andfocusing anode 13 is usually in the form of a coating as shown;electrical connection thereto being made by means of spring fingersmounted on electron gun 12 and insulated therefrom. A target 14 which isscanned by an electron beam generated by the electron gun 12, is

mounted adjacent to the opposite end of envelope 11. The target 14 mayalso be formed as part of the envelope itself but in any case is mountedso that object light passing through a camera lens system (not shown)may be readily directed thereupon. Terminal pins 15, sealed through theenvelope, afford convenient means for making electrical connection withand supporting target 14. Means are provided for scanning the electronbeam over the surface of target 14 and may include focus coil 16,deflection yoke 17, and alignment coil 18. To insure that the electronbeam at its final approach to the surface of target is normal thereto afine mesh screen electrode 19 is mounted adjacent to target 14. One ofthe terminal pins is provided for making electrical connection withelectrode 19.

Referring now to Figures 2 and 3, target 14 includes means forsupporting various films or coatings and may be a transparent supportingsheet 20, such as glass, having a plurality of spaced conductors or busbars 21 formed thereon. A bus bar 21 is provided for each of the videosignals. Preferably two sets of bus bars are formed one adjacentopposite ends of sheet 20. Color filter strips 22 extend acrosstransparent insulating sheet and are color filters that pass the desiredcolors such as red, green and blue. As described in detail in the abovereferred to patent of P. K. Weimer, the color filter strips 22 areformed by evaporating successive layers of high and low index ofrefraction material. The materials utilized namely zinc selcnide andcryolite or zinc sulphide and cryolite form insulating films when laiddown.

Transparent conductive strips 23am formed one on each of the filterstrips 22. Transparent conductive strips 23 are each somewhat longerthan filter strips 22 and extend over and make electrical contact withrespective bus bars 21. Conveniently on one side red filter strips 22extend over the blue and green bus bars and serve to insulate the redsignal strips therefrom. Similarly green filter strips 22 extend overthe blue bus bar 21 and afford similar insulation between each greensignal strip and the blue bus bar. On the opposite side of transparentsheet 20 the order of the bus bars 21 from the inside out is seen to bereversed. Intermediate each of the conductive signal strips 23 and thebus bar 21 with which it makes contact there is a short strip or finger25 formed by evaporating conductive material through a suitably formedand registered mask. The reactive impedance between each strip and itsbus bar is substantially reduced when such materials as aluminium orgold are used to form the fingers. Preferably I utilize silver ormaterial having similar properties in forming fingers 25. I have foundthat because of properties characteristic of silver, fingers '25 notonly reduce the reactive impedance of the connections between signalstrips 23 and bus bars 21 but also make them substantially uniform andhighly conductive. As shown, fingers 25 are longer than bus bars 21 arewide.

Ray sensitive material 24 in the form of a coating is applied totransparent conductive strips 23. Coating 24- is substantially thickerthan any of the other strips or coatings formed on transparent sheet 20and is so indicated in the drawing. I11 a photo-conductive pickup tubesuch as tube 10, photo-conductive porous antimony sulphide may beutilized; such material being described in the U. S. Patent 2,744,837 ofS. V. Forgue, filed June 1, 1951, and assigned to the assignee of thisapplication.

As pointed out above, the various coatings and strips may be formed byevaporating suitable materials onto transparent sheet 20. In practicebus bars 21 are formed preferably by evaporating such material as goldthrough a suitable mask to form relatively thick conductive lines whichare spaced apart as shown. While bus bars 21 are described as beingrelatively thick they are nevertheless non-self-supporting coatings andgenerally of such a thickness as not to be transparent. Fingers 25 arethen laid down located so as to extend intermediate each signal stripand its bus bar. Silver fingers 25 are formed by evaporating silverthrough a mask about 500 angstrom units thick. The various color filterstrips are formed by evaporating successive layers through a wiremasking grill. Color filters having the desired pass band may thus beformed. To form the red filters successive layers of zinc sulphide andcryolite are evaporated. Each of the layers has a thicknesscorresponding to an optical thickness of a quarter wave length of lightof approximately 4300 angstrom units in wave length. The blue filtersare formed with the same materials with layers of and optical. thicknessof a quarter wave length of light of about 5730 angstrom units in wavelength. Sulficient layers are formed so as to obtain a filter of thedesired efficiency. For example, eleven layers have proved satisfactoryin practice. The green color filters are formed by evaporatingsuccessively a three quarter wave englh optical thickness of zincselenide and a quarter wave length thickness of cryolite of light ofapproximately 6500 angstrom units in Wave length. Again sufficientlayers are formed, such as nine, so as to establish an efficient greenfilter.

Transparent conductive strips 23 are preferably formed from gold asdescribed in my application Serial No. 344,615, filed simultaneouslyherewith and assigned to the assignee of this application. The goldsignal strips are extremely thin and are estimated to range in thicknessfrom approximately 50 to angstrom units.

From the foregoing it is apparent that color filter strips 22 are alsoextremely thin though relatively thicker than signal strips 23. I havefound that during the evaporating of color filters 22, material isscattered by the wires of the mask grill into the spaces on transparentsheet 20 lying behind the wires of the grill. This scattering of thefilter material does not interfere with the formation of good filters.However, I have found that a film of filter materials of approximately ahalf wave length in thickness is formed over at least some of the busbars where it is necessary for signal strips 23 to make contact. Thus,at least some of the signal strips are separated from bus bars 21 by athin film of insulating material of a thickness of about /2 wave length.Thus, resistance measurements made after processing of targets withoutsilver fingers indicated those signal strips were completely insulatedfrom their bus bars as to direct current potentials. While such aconnection affords capacitive coupling I have found the impedance ascompared to the reactive impedance between the different sets of signalstrips to be too high in the absence of fingers 25. Since the conductivefingers 25 are longer than the bus bars 21 are wide, the signal strip tobus bar capacitance is increased with a consequent reduction in reactiveimpedance. When the short strips or fingers 25 are of silver they notonly cause a reduction in the reactive impedance, but also serve torender the nonconductive insulating films conductive. While the specificnature of this phenomenon is not completely understood at this time, itis believed that the silver fingers 25 act in some way to activate atleast one of the filter materials, probably the zinc selenide, to renderthe otherwise insulating film conductive.

Targets 14 have uniform low impedance connections between signal strips23 and bus bars 21. Substantially uniform resistances as low asapproximately 100 ohms, the resistance of the strips themselves, havebeen obtained measuring from bus bar to bus bar along the signal stripsas compared to 10,000 to 1,000,000 ohms or more in the past. The lowunit impedance along the signal strips to their bus bars and the outputcircuit substantially eliminates cross talk between adjacent signalstrips and the resulting mixing of colors. Furthermore, signal stripshaving a break along their length may yet produce a uniform color signalin view of the uniform connection at opposite ends thereof to the busbars 21. Another advantage results from the fact that breaks along thebus bars themselves are no longer troublesome as heretofore inasmuch assuch breaks are shunted by the many parallel low impedance paths of therespective signal strip sets. The possibility of streaks appearing inthe color picture at the receiver is also reduced since all of thestrips of each set are thus uniformly connected by a low resistanceconnection to their bus bars.

From the foregoing it is apparent that I have provided an improved tubeand target particularly suitable as a color television pickup tube.While I have described my invention in detail in connection with thephoto-conductive pickup tube shown for purposes of illustration, myinvention is applicable to other types of tubes. For example, myinvention is applicable also to cathode ray tubes in which ray sensitivematerial of the electron emissive or light emissive type is used on thetarget. In some tubes where it is not necessary that light pass throughthe signal strips then the signal strips need not be transparent.

While I have described color filters which are insulating, other filterswhich are not insulating or even constructions in which some otherarrangement is utilized for separating the primary colors may be used inaccordance with this invention. In such case, insulation is requiredwhere signal strips of one set cross over bus bars of another set. Suchinsulation is formed by evaporating insulating fingers. Where aninsulating film is formed by scattering of material on bus bars atpoints where contact is to be made then fingers 25 of conductivematerial serve to reduce the impedance in spite of the presence of thenon-conductive film. Therefore, while various changes and deviations maybe made from the specific construction described herein withoutdeparting from my invention, it is intended that all such changes anddeviations that come within the scope of the appended claims be includedin my invention.

What is claimed is:

1. A cathode ray tube, comprising a ray sensitive target, means forprojecting toward said target an electron beam which may be scanned overthe target, a plurality of spaced conductive signal strips extendingacross said target, a plurality of bus bars extending across saidtarget, sets of said signal strips each being connected to one of saidbus bars and insulated from the other sets and from the bus barsconnected to the other sets, each of said signal strips having a portionextending over the bus bar connected thereto, and a plurality of fingerstrips of conductive material each extending intermediate at least oneof said portions and the bus bar connected thereto.

2. A cathode ray tube, comprising a ray sensitive target, means forprojecting toward said target an electron beam which may be scanned overthe target, a plurality of spaced conductive signal strips extendingacross said target, a plurality of bus bars extending across saidtarget, sets of said signal strips each being connected to one of saidbus bars and insulated from the other sets and from the bus barsconnected to the other sets, each of said signal strips having a portionextending over the bus bar connected thereto, and a plurality of fingerstrips of conductive material each extending intermediate at least oneof said portions and the bus bar connected thereto, said finger stripsbeing substantially longer than the width of said bus bars.

3. A cathode ray tube, comprising a ray sensitive target, means forprojecting toward said target an electron beam which may be scanned overthe target, a plurality of spaced conductive gold signal stripsextending across said target, a plurality of gold bus bars extendingacross said target, sets of said signal strips each being connected toone of said bus bars and insulated from the other sets and from the busbars connected to the other sets, each of said signal strips having aportion extending over the bus bar connected thereto, and a plurality ofsilver finger strips each extending intermediate at least one of saidportions and the bus bar connected thereto.

4. A pickup tube, comprising a ray sensitive target,

6 means for projecting toward said target an electron beam for scanningsaid target, a plurality of spaced conductive signal strips extendingacross said target, a plurality of spaced bus bars extending across saidtarget, a plurality of spaced bus bars extending across said target in adirection transversely of said signal strips, sets of said signal stripseach being connected to one of said bus bars and insulated from theother sets and from the bus bars connected to other sets, each of saidsignal strips having a portion thereof extending over the bus barconnected thereto, and a plurality of conductive finger strips eachextending intermediate at least one of said portions and the bus barconnected thereto.

5. A pickup tube, comprising a ray sensitive target, means forprojecting toward said target an electron beam for scanning the target,a plurality of parallel color filter strips extending across saidtarget, a plurality of spaced parallel conductive signal stripsextending across said target one in registration with each of said colorfilter strips, a plurality of bus bars extending across said target in adirection transversely of said strips, sets of said signal stripsassociated with filters of the same color each being connected to one ofsaid bus bars and insulated from the other sets and from. the bus barsconnected to the other sets, and a plurality of conductive finger stripseach extending intermediate at least one of said signal strips and thebus bar connected thereto.

6. A pickup tube as described in claim 5 wherein said finger strips aresilver.

7. A pickup tube as described in claim 5 wherein said bus bars and saidsignal strips are gold and said finger strips are silver.

8. A pickup tube for color television, comprising a photosensitivetarget, means for projecting toward said target an electron beam forscanning the photosensitive surface of the target, a plurality ofinsulating color filter strips extending across said target, a pluralityof spaced conductive signal strips one on each of said filter strips, aplurality of bus bars extending across said target in a directiontransversely of said strips, signal strips on filter strips of a givencolor extending across and connected in parallel by one of said bus barsand insulated from the remaining signal strips and the bus barsconnected thereto, and a plurality of silver finger strips eachextending intermediate at least one of said signal strips and the busbar connected thereto.

9. A pickup tube for color television, comprising a photosensitivetarget, means for projecting toward said target an electron beam forscanning a surface of said target, a plurality of insulating colorfilter strips extending across said surface of the target, a pluralityof spaced conductive signal strips one on each of said filter strips, aplurality of bus bars extending across said target in a directiontransversely of said strips, signal strips on filter strips of a givencolor extending across and connected in parallel by one of said bus barsand insulated from the remaining signal strips and the bus barsconnected thereto, a plurality of silver finger strips each extendingintermediate at least one of said signal strips and the bus barconnected thereto, and a coating of photoconductive material on saidsignal strips.

10. A ray sensitive target for a cathode ray tube, comprising aninsulating supporting member, a plurality of spaced conductive signalstrips extending across said member, a plurality of bus bars on saidmember, sets of said signal strips each being connected to one of saidbus bars and insulated from the other sets and from the bus barsconnected to the other sets, and a plurality of conductive finger stripseach extending intermediate at least one of said signal strips and thebus bar connected thereto.

11. A ray sensitive target as described in claim 10 wherein said fingersare of highly conductive material providing connections between thesignal strips and bus bars that are highly conductive as to directcurrent potentials.

12. A ray sensitive target for a cathode ray tube, comprising aninsulating support member, a plurality of spaced conductive signalstrips extending across said member, a plurality of bus bars on saidmember, sets of said signal strips each being connected to one of saidbus bars and insulated from the other sets and from the bus barsconnected to the other sets, and a plurality of silver finger stripseach extending intermediate one of said signal strips and the bus barconnected thereto.

13. A photosensitive target for a color television pic'kup tube,comprising an insulating supporting sheet, a plurality of color filterstrips extending across said sheet in parallel relation, a plurality ofsubstantially transparent conductive spaced signal strips one on each ofsaid color filter strips, a plurality of bus bars extending across saidsheet in a direction transversely of said strips, the signal strips oncolor filter strips of the same color extending across one of said busbars and electrically connected in parallel by said bus bar and beinginsulated from the remaining signal strips and the bus bars connectedthereto, and a plurality of conductive finger strips each extendingintermediate at least one of said signal strips and the bus barconnected thereto.

14. A photosensitive target as described in claim 13 wherein said fingerstrips are of highly conductive material providing connections betweenthe signal strips and bus bars that are highly conductive as to directcurrent potentials.

15. A photosensitive target for a color television pickup tube,comprising an insulating support sheet, a plurality of color filterstrips extending across said sheet in parallel relation, a plurality ofsubstantially transparent conductive spaced signal strips one on each ofsaid color filter strips, a plurality of bus bars extending across saidsheet in a direction transversely to said strips, the signal strips oncolor filter strips of the same color extending across one of said'busbars and electrically connected in parallel by said bus bar and beinginsulated from the remaining signal strips and the bus bars connectedthereto, and a plurality of silver conductive finger strips eachextending intermediate one of said signal strips and the bus barconnected thereto.

16. A photosensitive target as described in claim 15 .comprisingphotoconductive material on said signal strips.

17. A ray sensitive target for a cathode ray tube, comprising aninsulating support member, a plurality of spaced conductive signalstrips extending across said member, a plurality of bus bars on saidmember, sets of said signal strips each being connected to one of saidbus bars, each of said sets of signal strips being insulated from theother sets and the bus bars connected to said other sets, and aplurality of conductive finger strips each extending intermediate one ofsaid signal strips and the bus bar connected thereto.

References Cited in the file of this patent UNITED STATES PATENTS2,446,791 Schroeder Aug. 10, 1948 2,577,368 Schultz et al Dec. 4, 19512,589,386 Huffman Mar. 18, 1952

